The present invention relates to a material for packaging light-sensitive materials which deteriorate on exposure to light, in particular, to a material for packaging light-sensitive materials for photographic use.
Heretofore, as the light-shielding paper for such packaging materials, for example as a backing sheet for photographic roll film, a composite sheet comprising a flexible substrate (a so-called "flexible sheet") such as paper coated with a thermoplastic resin layer has widely been used. A wide variety of resins have been used for the thermoplastic resin layer, all having both merits and demerits.
For instance, low density polyethylene (hereinafter referred to as "LDPE") when used for the thermoplastic resin layer has the following demerits: various kinds of defects due to its insufficient flexibility tends to develop; heat seal temperature is high; no heat-sealability with a polystyrene spool for photographic film is obtained; when LDPE is heat-sealed with LDPE, heat-seal defects due to dusts, powders etc. (i.e. when powdery concomitants are present, heat-sealability is decreased) often occur; addition of a light-shielding material decreases strength of film and sealing strength.
Similarly, to take the case of ethylene-vinyl acetate copolymer (hereinafter referred to as "EVA"), it has the following defects: water absorbability is too great; blocking tends to occur; carbon black is not uniformly dispersed; a substrate such as paper is not well adhered; neck-in is too great; film composed of EVA is not sharply and readily cut. Therefore, a composite sheet composed of a substrate coated with a single melt extruded EVA layer containing carbon black is not yet put to practical use as a light-shielding paper for the photographic roll film.
In case that acrylic ester copolymer such as ethylene-ethylacrylate copolymer (hereinafter referred to as "EEA") is coated through melt extrusion, there also arise problems similar to those with the EVA coating such as too great neck-in, frequently occurring blocking, uneven dispersion of carbon black, poor adhesion to a substrate (Generally, EEA is inferior to LDPE) and a high cost.
Consequently, most light-shielding paper has been manufactured by a process in which EVA is dissolved in a solvent and the resultant solution is coated to a substrate, which results in a high manufacturing cost. In case of EEA coating, the hot melt coating or the above-mentioned solution coating process has often been used for preparing a light-shielding paper for packaging powdered medicine or food such as chocolate, which leads to a high manufacturing cost. Since melt extrusion coating of EVA or EEA causes great neck-in and insufficient adhesion to a substrate, the solution coating process has been the only choice available.